That’s a late 20th century invention, and not universally followed. Consider Unix time functions, which predate the ISO standard and follow what’s common practice in America even today, that Sunday is the first day of the week. Of course, Sunday was the first day of the week in Europe for over a thousand years and that practice has not fully been extinguished.

In French. "End of week" would be "fin de semaine" but we don’t say that, we say "weekend". Which is understood as these two days when most people don’t work.

If anything, "fin de semaine" would refer to the last working days before the weekend.